SPECIAL FORMS OF FEOST SCREWS. 



163 



Amongst them we may cite, firstly, the screws and cogs with 

 an H-formed surface (fig. 127). 2. Screws with a cross-shaped 

 bearing surface (fig. 128). 3. Screws and cogs with T-formed 



Fig. 127. — Screw with H-shaped head. 



Fig. 12S.— Screw with +-shaped head. 



ground surface. 4. Screws and cogs with S-shaped ground 

 surface. 5. Angled screws (fig. 129). 6. Screws and cogs 

 with inserted rubber plugs. 7. Screws with radiating Y-shaped 



Fig. 129. 



Fig. 130. 



Fig. 129.— Screw with angled head. 

 Fig. 130.— With Y-sliaped head. 



Fig. 131. Fig. 132. 



Fig. 131. — Hollow screw. 

 Fig. 132. — Perforated screw. 



bearing surface (fig. 130). 8. Hollow screws (fig. 131). 

 9. Perforated screws (fig. 132). And 10. Spring cogs. 



The durability of these screws and the foothold which they 



Fig. 133. -Universal screw-key. 



give depend chiefly on their diameter and the direction of the 

 wearing surfaces. So far as experience teaches, those with the 

 narrowest and fewest wearing surfaces are least durable, but this 



